How i can make a shop for skybox/background?

You could use Scriptable Objects to store player/shop data.

https://docs.unity3d.com/Manual/class-ScriptableObject.html https://learn.unity.com/tutorial/introduction-to-scriptable-objects

 using UnityEngine;

 [CreateAssetMenu(fileName = "PlayerData", menuName = "Data/PlayerData")]
 public class PlayerDataScriptableObject : ScriptableObject
 {
     public int diamonds;
 }

Then apply the same logic to store data about if the player "owns" a particular skybox.

 using UnityEngine;

 [CreateAssetMenu(fileName = "ShopData", menuName = "Data/ShopData")]
 public class ShopDataScriptableObject : ScriptableObject
 {
     public bool skybox56;
 }

and then have some kind of shop UI with a button that allows them to purchase the skybox.

 public sealed class ShopManager : MonoBehaviour
 {
     public PlayerDataScriptableObject playerData;
     public ShopDataScriptableObject shopData;

     public Button skybox56;

     void Start()
     {
         skybox56.onClick.AddListener(BuySkybox56);
     }

     private void BuySkybox56()
     {
         if (playerData.diamonds > 100)
         {
             shopData.skybox56 = true;
             playerData.diamonds -= 100;
         }
     }
 }